Turbine rotors have wheels provided with dovetail constructions, referred to as dovetails, at which point the wheel buckets are attached. The dovetails require inspection or examination for flaws. Standard inspection procedures for the dovetails have involved rotating the rotor. Rotating the rotor is a conventional part of the procedure for inspecting non integral rotors, i.e., rotors where the turbine wheels have been shrunk on the rotor shaft. That is, the inspection of the shrink interface is done while rotating the rotor, which is usually achieved by mounting and spinning the rotor on power roller equipment. Inspecting the dovetails of non-integral rotors by a procedure that involves rotating the rotor is readily facilitated since the rotor rotating equipment is available.
Automated ultrasonic test systems for detecting flaws in turbine wheel dovetails for use in conjunction with the rotation of the rotor by the power roller equipment have been developed. In general, these test systems include an optical encoder placed on either the turbine rotor shaft or the power roller equipment and the use of position data derived from the optical encoder for triggering ultrasonic pulses from a transducer at regular intervals. The ultrasonic test system acquires the data in polar format and plots signals in a B-scan display.
In the above described wheel dovetail inspection system the ultrasonic transducer is required to be positioned at a precise radial location on the wheel and remain at that location throughout 360 degrees rotation of the turbine rotor. A fixed manipulator or transducer holder with several axes of motion for alignment purposes is employed for precisely locating the transducer.